Digested sludge is known to consolidate less readily than most primary sludges. The latter will usually consolidate in conventional primary sedimentation tanks to provide a sludge containing between 4 and 6% dry solids. This degree of consolidation is achieved with considerably less than one day's settlement. Anaerobic digestion of such a primary sludge results in up to 50 percent of the organic matter being rendered soluble or released as sludge gas. A typical digested sludge contains only 2.5% dry solids after 20 days in a primary digester.
Frequently, the primary digester is coupled in series with a secondary `digestion` or thickening tank where the sludge may be held for a further 10 to 60 days. Traditionally, this secondary digester is similar to the primary digester though it is neither heated nor mixed as is the primary digester. In the secondary digester, gravity concentration of digested sludge solids and decanting of supernatant liquor takes places. This reduces the volume of the sludge requiring further processing and disposal. Very little solids reduction and gas production takes place in the secondary digester.
Unfortunately, many secondary digesters have performed poorly as thickeners, producing dilute sludge and a high strength supernatant liquor. The basic cause of the problem is that, in most cases, anaerobically digested sludges do not settle readily. Two major factors contribute to this phenomenon:
(1) Continued gas evolution: During the digestion process considerable quantities of sludge gas, typically containing 70% methane and 30% carbon dioxide, are produced. When fresh digested sludge is pumped into large thickening tanks the sludge is still warm and can continue the digestion process, producing sludge gas for many weeks. This results in rising gas bubble disturbance of the sludge, preventing any significant consolidation until gas production ceases.
(2) Microbubble hold up: The contents of the primary digestion tank become supersaturated with methane and carbon dioxide gas. When this sludge is transferred into the secondary digestion tank, the gas comes out of solution, forming small bubbles. These microbubbles attach to sludge particles and provide a bouyant force that hinders settling until the bubbles coalesce and rise to the sludge surface.
(3) The effect of continued gas evolution particularly in large volume, deep secondary thickeners is to prevent any separation of supernatant liquor and sludge for periods of up to 30 days. Even after gassing ceases, settlement is still extremely slow, in deep (10 m), secondary thickeners, moving to the pronounced microbubble hold up effect.
The problem of this lack of consolidation, or thickening, of this digested sludge will be apparent when its handling, and volume is considered.
The quantity of sewage sludge disposed of annually in the United Kingdom amounts to about 35 million wet tonnes of which about 50% is digested. The problem of consolidating the 18 million m.sup.3 of liquid sludge produced by anaerobic digestion prior to ultimate disposal to land or sea is thus of great importance.
Consolidation or thickening of all digested sludge to at least 5% solids, could reduce the UK annual sludge disposal volume by 5 to 9 million m.sup.3. Liquid sludge disposal transport costs were at least 1 to 2 per m.sup.3 in 1982 for typical sea and land disposal operations. Annual transport savings of several million could therefore be made if the 5% solids target could be reliably attained at all UK sludge sites.